Process for the preparation of reinforced rubber vulcanizates and use thereof

ABSTRACT

A process for the preparation of reinforced vulcanizates which process comprises vulcanizing a mixture comprising rubber, a novolak and at least one reactive melamine resin obtained by reaction of one mol of melamine with 0.5 to 6 mols of aldehyde, said melamine resin being present in an amount of from 0.5 to 120% by weight, based on the weight of the novolak resin, and the phenolic component of said novolak resin being selected from the group consisting of phenol, polynuclear polyphenols, alkylphenols, a combination of alkylphenols with phenol, and a combination of at least one of said phenols with a further phenol selected from the group consisting of resorcinol, phenylphenol and polyalkylphenols. The invention also includes rubber articles prepared from a product of such a process.

PRIOR APPLICATION

This application is a continuation of copending U.S. patent applicationSer. No. 269,477 filed June 2, 1981 which in turn is a continuation ofU.S. patent application Ser. No. 098,765 filed Nov. 30, 1979, both nowabandoned.

It is known to prepare light rubber grades from natural or syntheticrubber by vulcanization with sulphur with the addition of at least 0.5%(relative to the weight of the rubber) of a curable etherifiedcondensation product of aldehydes and urea and/or melamine orderivatives thereof. The melamine resins used for reinforcement may beemployed as solid powdered condensation products.

It is also known to prepare hard rubber vulcanizates by thevulcanization of rubber mixtures with sulphur with the addition ofhexamethoxymethylmelamine or resins obtained from melamine or guanaminesand aldehydes the alkylol groups of which resins have been etherified byalcohols. The hardening effect obtained in the lastmentioned process maybe intensified by the addition of polyester resins formed fromdicarboxylic acids and polyhydric alcohols and having an acid number ofat least 50.

According to a further publication it is known that mixtures of naturalor synthetic rubber may be reinforced by the addition of novolaks andresols in a ratio of 9:1 to 1:9.

The above-mentioned processes can be carried out according to thespecifications mentioned and produce the effects cited therein, but theyhave assumed only limited significance economically.

However, the reinforcement of rubber grades of the most diverse types bythe addition of phenol novolaks and hexamethylene tetramine as curingagent is of practical importance. The reinforcing effect of the resinsused may if desired be effectively controlled by the quantity of curingagent employed.

Although this method of reinforcing rubber using a novolak resin andhexamethylenetetramine is successful in practice, it has recently beencriticised on the grounds of environmental safety, sincehexamethylenetetramine irritates the human skin and is said to give riseto rashes.

It has surprisingly been found that a large variety of novolaks suitablefor the reinforcement of rubber may be cured by the addition of reactivemelamine resins, and in this way especially efficient reinforcingeffects may be achieved. The invention is therefore concerned with aprocess for the preparation of reinforced vulcanizates by thevulcanization of natural or synthetic rubber mixtures with novolakresins in the presence of an amine compound as curing agent, which ischaracterised in that the vulcanization is effected in the presence of,as curing agents, reactive melamine resins obtained by reacting melaminewith 0.5 to 6 moles of aldehyde, in a proportion of 0.5 to 120,preferably 2 to 50% by weight, relative to novolak resin, and in thatthe phenolic component of the novolak is selected from the group phenol,polyhydric polynuclear phenols, alkyl phenols, mixtures of alkyl phenolswith phenol, or a combination of at least one of said phenols with afurther phenol selected from the group resorcinol, phenylphenol orpolyalkyl phenols. In the latter case the further phenols may beemployed in such a quantity that the molar ratio of the total quantityof the further phenols to the total quantity of the other phenols is upto 1:1, advantageously not more than 1:2.

The methylol groups present in the melamine resin may be free or atleast partly etherified by reaction with monohydric alcohols with 1 to12 carbon atoms.

Melamine resins alone, used according to the above-mentioned knownprocesses, have a distinctly weaker effect than the conventionally usednovolak/hexamethylene tetramine systems in rubber mixtures of varioustypes. When solid reactive melamine resins are employed in suchprocesses, their effect depends largely on the fineness of the particlesthereof. In practice, the particle size of active fillers is achievedonly in exceptional cases.

Solid melamine resins may also optionally be used in the processaccording to the invention. As a rule, a malamine resin with a particlesize of at most 100 μm, preferably about 50 μm will be used. Highlyreactive melamine resins can be prepared in solution relatively easily.Resins of this type are suitable for working into rubbers only to alimited degree, since the required solvents such as higher alcohols,water, polyglycols or esters are not compatible with most rubber types.They have to be evaporated during the mixing process. Althoughhexamethoxymethylmelamine itself is easy to work in, it cures onlyslowly. For this reason, it does not have the effect of the phenolnovolak/hexamethylene tetramine mixtures used in practice.

Hexamethoxymethylmelamine alone reacts with itself only very slowly uponheating. If a reaction is to take place at room temperature, the use ofthis substance necessitates the addition of strong acids, e.g.hydrochloric acid or toluenesulphonic acid. For this reason, it issurprising that in the process according to the invention the rubbermixtures to which phenol novolaks of the most diverse types have beenadded for reinforcement are cured very efficiently by melamine resins ofthe above-described type, especially hexamethoxymethylmelamine or thecorresponding higher polynuclear condensation products of monomericmelamine resins. A reinforcing effect many time more intense even thanthat obtained with novolak/hexamethylene tetramine mixtures can beachieved. The reinforcing effects achieved with the process according tothe invention are particularly well illustrated by the increase in thestress values at 100, 200 and 300% extension as well as by the increasein the vulcanizate hardness.

In applications of the process according to the invention the speed ofcure and the speed of the hardening effect may be substantiallyaccelerated and intensified by the addition of suitable monobasic ordibasic organic acids with at least 2 and at most 20 carbon atoms, suchas phthalic, benzoic, maleic, fumaric, trimellitic and pyromelliticacids and anhydrides of such acids, or sulphonic acids such asnaphthalenedisulphonic acid or sulphanilic acid.

In addition, polyester resins formed from polycarboxylic acids andpolyhydric alcohols with acid numbers of 20 and above, (e.g. up to 100)may be used instead of or together with the organic acids, and suchresins may also be modified by monocarboxylic acids and monohydricalcohols. However, the proportion of monohydric components should amountat most to 25 equivalent % relative to the acid or alcohol component.

The organic acids or polyester resins serving to accelerate curing maybe added in quantities of 0.1 to about 5, preferably 1.5 to 3% byweight, relative to rubber. Larger or smaller amounts of acid may alsobe employed.

The acceleration of curing may also be achieved by means of stronginorganic acids such as phosphoric acid or acidic phosphoric esters.However, this method is inconvenient due to the precautionary measuresnecessary when such acids are used.

The starting mixtures employed according to the invention may beprepared by conventional means in internal mixers or on mixing rolls. Itis necessary to ensure when the resins are worked into the rubber thatthe temperature of the mixture exceeds the melting point of the resins,especially that of the novolak resins, at every stage of thepreparation, so that the resins are distributed completely throughoutthe mixture. Furthermore, it is important that the melamine resins to beused as curing agents are worked into the mixture in such a way thepremature self-condensation or a premature reaction with the novolakwhich is to be cured do not occur.

Thus it is possible for such premature reactions to be avoided if themelamine resins are worked in near the end of the mixing operation attemperatures which are not too high. The appropriate temperature dependson the type of melamine resin used, on the quantity of acid and on thetime, and is generally between 80° to 120° C. In certain cases it may behigher, e.g. where hexamethoxymethyl melamine is used in the absence ofacid, or it may be lower.

Suitable types of rubber are, for example, natural rubber, styrenerubber, acrylonitrile rubber, polybutadiene, butyl rubber,ethylene/propylene/diene/terpolymer rubber or mixtures thereof, such asare used conventionally in the tire industry or in the manufacture ofindustrial rubber goods.

Suitable novolak resins are, for example, those obtained from phenol,polyhydric polynuclear phenols such as bisphenols, for examplediphenylol propane or diphenylolmethane, or mixtures of phenol and alkylphenols such as tertiary butyl phenol, octyl phenol or nonyl phenolreacted with aldehydes, preferably formaldehyde under acid conditions.Moreover, the novolak resins may contain softening agents such aspolyethylene glycols or tall oil or other suitable plasticising agents.In principle, novolak resins which consist exclusively of alkyl phenolscarrying an alkyl group of 1 to 12 carbon atoms may also be used; butthese are of only limited use since they are less readily cured.Furthermore, the novolak resins may contain additions of resorcinol orphenylphenol or phenols containing several alkyl groups. For practicaluse preferred novolak resins are those formed from phenol and mixturesof phenol and cashew-nut shell oil as well as from phenol and alkylphenols having 4-12 carbon atoms in the alkyl moiety, especially p-tert.butyl-, octyl-or nonylphenol. The proportion of substituted phenols inthe novolak may vary, as long as the novolak is still sufficientlycurable. Thus, a proportion of alkylphenol of at most 70 molar % of thetotal phenol component will generally be chosen.

The proportion of novolak relative to the elastomer is, as a rule, 1-30,preferably 3-20% by weight. In certain cases larger quantities e.g. 100%by weight and more relative to the elastomer may be used. With such ahigh propportion of novolak hard products are obtained which differsubstantially in their properties from the conventional high-elasticityrubber vulcanizates.

Preferred melamine resins are hexamethoxymethyl melamine orcorresponding more highly condensed polynuclear products or at leastpartly etherified trimethylol-, tetramethylol- or pentamethylol-melamine resins. The degree of condensation of the aldehyde resins,especially of the formaldehyde resins and their content of free methylolgroups may be controlled in a conventional way by correspondingadjustment of the reaction medium. The free methylol groups may beetherified with monohydric alcohols having 1-12, preferably 1-8 carbonatoms, such as methanol, the various isomers of butanol, ethylhexylalcohol, n-octanol, nonanol and dodecyl alcohol.

The optimum quantities of melamine resins necessary for curing thenovolak resins may be determined, in general, in simple preliminarytests. In general, about 1-80, preferably 2-50% by weight of melamineresin is required per 100 parts by weight of novolak resin.

It is also possible to add, instead of or together with the acids,softeners conventional in rubber processing, for example aromaticpolyethers, phthalates and the like.

Commercially obtainable active or inactive carbon black, silicic acids,kaolins, chalks or other conventional materials may be used as fillers.As a rule, sulphur, in conjunction with known accelerators, is used forvulcanization. In many cases, however, the processes may be carried outwithout the use of sulphur. Vulcanization in the presence of sulphur is,nevertheless, preferred. The course of vulcanization may be adapted topractical requirements by a choice of suitable accelerators conventionalin rubber technology.

Industrial rubber articles may be manufactured from the mixturescontaining a novolak resin and a melamine resin prepared according tothe invention. However, they may be used also for the manufacture ofmotor vehicle tires.

In the following Examples all quantities are expressed in parts byweight unless otherwise stated.

EXAMPLES

                                      TABLE 1                                     __________________________________________________________________________                    V1    V2                                                                      (Compari-                                                                           (Compari-                                               Mixture No.     son)  son)  1   2   3   4                                     __________________________________________________________________________    Styrene-butadiene                                                                             100   100   100 100 100 100                                   rubber.sup.(1)                                                                Carbon black N-330.sup.(2)                                                                    50    50    50  50  50  50                                    Stearic acid    1.5   1.5   1.5 1.5 --  --                                    ZnO             5     5     5   5   5   5                                     Age-resisting agent                                                                           2     2     2   2   2   2                                     Sulphur         --    --    2   2   2   2                                     Phenol novolak.sup.(3)                                                                        --    18    14  14  14  14                                    Hexamethylene tetramine                                                                       --    2     --  --  --  --                                    Hexamethoxymethyl                                                                             --    --    6   6   6   6                                     melamine.sup.(4)                                                              Benzoic acid    --    --    --  3   --  --                                    Diethylene glycol                                                                             --    --    --  --  3   --                                    Acid phthalic acid/penta-                                                                     --    --    --  --  --  3                                     erythritol/polyester resin                                                    Sulphenamide accelerator                                                                      1.2   1.2   1.2 1.2 1.25                                                                              1.25                                  Tetramethyl thiuram mono-                                                                     0.2   0.2   0.2 0.2 --  --                                    sulphide                                                                      Zinc-N--diethyl dithio-                                                                       --    --    --  --  0.25                                                                              0.25                                  carbamate                                                                     Vulcanisation 150° C. 20 minutes                                       Tearing strength (MPa)                                                                        25.6  22.2  25.0                                                                              22.5                                                                              21.9                                                                              23.4                                  Elongation at break (%)                                                                       315   314   357 360 303 341                                   Stress value at 100%                                                                          5.3   6.6   7.0 8.5 7.2 6.6                                   extension (MPa)                                                               Stress value at 200%                                                                          14.4  14.7  14.0                                                                              15.8                                                                              14.7                                                                              13.8                                  extension (MPa)                                                               Stress value at 300%                                                                          24.1  21.6  21.3                                                                              23.0                                                                              21.3                                                                              21.3                                  extension (MPa)                                                               Shore Hardness (Shore A)                                                                      71    84    88  90  86  87                                    __________________________________________________________________________     Footnotes                                                                     .sup.(1) Styrenebutadiene rubber containing resin fatty acid with 23.5%       bound styrene.                                                                .sup.(2) Carbon black for high abrasion resistance. A rubber/carbon black     master batch of 100 parts by weight of rubber and 50 parts by weight of       carbon black was used.                                                        .sup.(3) Commercial phenol novolak, m.p. 83-88° C. (capillary          method).                                                                      .sup.(4) Viscosity as supplied: 8-22 Pa.s at 20° C.               

DISCUSSION OF THE RESULTS OF TABLE 1

Mixtures V 1 and V 2 are comparison mixtures. Mixture V 2 shows thereinforcement of a styrene/butadiene/carbon black mixture with acommercial phenols novolak upon hardening with hexamethylenetetramine.Mixture 1 shows the reinforcing effect according to the invention uponthe use of a commercial hexamethoxymelamine grade free of water andsolvent and adjusted for high viscosity. Mixture 2 shows the effect ofbenzoic acid used to accelerate the hardening.

The vulcanisates of mixture V 2 differ from those of mixture V 1 inhaving a rather higher stress value at 100% and a clearly highervulcanisate hardness. Mixture 1 prepared according to the inventiongives vulcanisates with a further increased stress value at 100%extension and much higher hardness. With the use of benzoic acid thestress values at 100, 200 and 300% extension and the vulcanisatehardness are also increased.

If a material of lower viscosity (4,000-8,000 mPa.s) is used instead ofthe high-viscosity hexamethoxymethylmelamine, vulcanisate hardnessesabove 90° Shore A can be obtained.

If 6 parts by weight of a powdered melamine resin obtained by reacting 1mole of melamine with about 1.2 moles of formaldehyde are used insteadof the 6 parts by weight of hexamethoxymethylmelamine, then vulcanisatesare obtained which largely correspond in their test data to thevulcanisates of mixture 1.

Mixtures 3 and 4 prepared according to the invention show thatplasticising additives such as diethylene glycol or an acidic phthalicacid/pentaerythritol/polyester resin may be used with advantage. Due tothe diethylene glycol the Mooney viscosity is reduced by about 10 to 15units in relation to a comparison mixture. The polyester resin added tomixture 4 (which has an acid number of about 200) assists curing.

                                      TABLE 2                                     __________________________________________________________________________    Mixture No.    V5  6   7   8   9   10  11                                     __________________________________________________________________________    Styrene-butadiene rubber                                                                     100 100 100 100 100 100 100                                    Carbon Black N-330                                                                           50  50  50  50  50  50  50                                     Stearic acid   1.5 1.5 1.5 1.5 1.5 1.5 1.5                                    ZnO            5   5   5   5   5   5   5                                      Age-resisting agent                                                                          2   2   2   2   2   2   2                                      Sulphur        2   2   2   2   2   2   2                                      N--cyclohexyl-2-benzo-                                                                       1   1   1   1   1   1   1                                      thiazole sulphenamide                                                         Tetramethyl thiuram mono-                                                                    0.2 0.2 0.2 0.2 0.2 0.2 0.2                                    sulphide                                                                      Dithiodimorpholine                                                                           --  --  --  --  --  2.5 --                                     Phenol novolak.sup.(1)                                                                       --  14  14  15  14  14  14                                     Trimethylol melamine                                                                         --  6   6   --  --  --  --                                     Pentamethylol melamine                                                                       --  --  --  5   --  --  --                                     resin.sup.(2)                                                                 Pentamethylolmelamine                                                                        --  --  --  --  6   6   --                                     trimethyl ether                                                               Tetrabutoxydimethoxymethyl                                                                   --  --  --  --  --  --  6                                      melamine.sup.(3)                                                              Benzoic acid   --  --  3   1.5 --  --  --                                     Phthalic acid  --  --  --  --  3   3   2                                      Vulcanisation 145° C. (in a                                                           60  60  60  60  30  30  30                                     press) min.                                                                   Tearing strength (mPa)                                                                       19.4                                                                              20.6                                                                              19.4                                                                              22.2                                                                              20.3                                                                              19  18.8                                   Elongation at break (%)                                                                      276 286 266 275 262 245 256                                    Stress value 100% (mPa)                                                                      --  --  --  --  6.6 7.5 6.3                                    Stress value 200% (mPa)                                                                      13.8                                                                              14.1                                                                              16.3                                                                              16.6                                                                              16.3                                                                              19.2                                                                              14.4                                   Shore Hardness (Shore A)                                                                     72  80  84  82  86  90  87                                     Notch impact strength (N/mm)                                                                 10  9.8 10  9.9 --  --  12.5                                   __________________________________________________________________________

                  TABLE 3                                                         ______________________________________                                        Mixture No.      V12       13      14                                         ______________________________________                                        Phenol-butadiene rubber                                                                        100       100     100                                        Carbon Black N-330                                                                             50        50      50                                         Stearic acid     2         2       2                                          ZnO              4         4       4                                          Age-resisting agent                                                                            2         2       2                                          Sulphur          2         2       2                                          N--tert.-butyl-2-benzo-                                                                        1.2       1.2     1.2                                        thiazol sulphenamide                                                          Zinc diethyldithio-                                                                            0.25      0.25    0.25                                       carbamate                                                                     Phenol novolak   --        14      14                                         Tetramethylol melamine                                                                         --        11      --                                         dibutoxy resin.sup.(4)                                                        Pentamethylol melamine                                                                         --        --      8                                          trimethyl ether, modified                                                     with triethylene glycol                                                       Vulcanisation 30 min. 150° C. (in a press)                             Tearing strength (MPa)                                                                         26.3      20.3    22.5                                       Elongation at break (%)                                                                        297       281     270                                        Stress value 100% (MPa)                                                                        5.6       7       7.8                                        Vulcanisate hardness                                                                           72        88      87                                         Shore A (°)                                                            ______________________________________                                         Footnotes:                                                                    .sup.(1) Commercial novolak, m.p. 106° C. (ring and ball ASTM E        28-67).                                                                       .sup.(2) Commercial melamine resin, dynamic viscosity (20° C.)         4000-8000 mPa.s, dilutable in water as desired.                               .sup.(3) Commercial melamine resin, viscosity (20° C.) 900-1800        mPa.s.                                                                        .sup.(4) Commercial melamine resin, 55% in isobutanol, dynamic viscosity      (supplied form) 300-800 mPa.s. The resin was used with the solvent largel     excluded.                                                                

                  TABLE 4                                                         ______________________________________                                        Mixture No.          15      16                                               ______________________________________                                        Styrene-butadiene rubber                                                                           100     100                                              Carbon Black N-330   50      50                                               Stearic acid         1.5     1.5                                              ZnO                  5       5                                                Age-resisting agent  2.2     2.2                                              Sulphur              2       2                                                Cyclohexylbenzothiazol-                                                                            1.1     1.1                                              sulphenamide                                                                  Tetramethylthiuram   0.2     0.2                                              monosulphide                                                                  Phenol novolak.sup.(5)                                                                             14      7                                                Resorcinol novolak.sup.(6)                                                                         1.7     7                                                Hexamethoxymethyl melamine                                                                         6       6                                                Benzoic acid         2       3                                                Vulcanisation 40 min. 150° C. (in a press)                             Tearing strength (MPa)                                                                             18.8    19.1                                             Elongation at break (%)                                                                            215     243                                              Stress value 200% (MPa)                                                                            17.5    15                                               Hardness (Shore A) (°)                                                                      87      87                                               ______________________________________                                         .sup.(5) Commercial phenol novolak m.p. 96° C. (ring and ball ASTM     E 28-67).                                                                     .sup.(6) Commercial resorcinol novolak m.p. 65° C. (ring and ball      ASTM E 28-67).                                                           

The same types of rubber and carbon black were used respectively inTables 2 to 4 as in Table 1.

DISCUSSION OF THE RESULTS OF TABLES 2 to 4

Mixtures V5 and V12 are comparison mixtures. As shown by the comparisonof the hardness and stress values of mixtures 6 to 8 according to theinvention in Table 2 with the corresponding values of comparison sampleV5, the hardness and stress values have increased in relation to thoseof the comparison sample. Mixtures 9 to 11 show a greater increase ofthe hardness and stress which is presumably caused by the combinedeffect of the two melamine resins.

In Table 3 the mixtures 13 and 14 according to the invention showlikewise a clear improvement of the hardness and stress value inrelation to comparison sample V12. The higher hardness is presumablyattributable to the different accelerator system.

Table 4 indicates for both samples 15 and 16 a reinforced vulcanisatewith good stress values and high hardness.

It is not intended that the examples given herein should be construed tolimit the invention thereto, but rather they are submitted to illustratesome of the specific embodiments of the invention. Resort may be had tovarious modifications and variations of the present invention withoutdeparting from the spirit of the discovery or the scope of the appendedclaims.

We claim:
 1. A process for the preparation of reinforced vulcanizateswhich comprises vulcanizing a mixture comprising rubber, sulfur as avulcanization agent in conjunction with a conventional accelerator and anovolak as a reinforcing material in conjunction with at least onereactive melamine resin obtained by reaction of one mol of melamine with0.5 to 6 mols of aldehyde, said melamine resin being present in anamount of from 1 to 80% by weight based on the weight of the novolakresin, the novolak resin being present in the final composition in anamount of from 1 to 30% by weight based on the rubber, and the phenoliccomponent of said novolak resin being selected from the group consistingof phenol, bisphenols, alkylphenols and, a combination of alkylphenolswith phenol having a content of at most 70 mol % of alkyl phenol, andthe rubber being derived from components free of carbonyl groups.
 2. Aprocess as claimed in claim 1 wherein the vulcanisation is performed inthe presence of a melamine resin the methylol groups of which are atleast partially etherified with a monohydric alcohol having from 1 to 12carbon atoms.
 3. A process as claimed in claim 1 wherein said melamineresin is present in an amount of from 2 to 50% by weight based on theweight of the novolak resin.
 4. A process as claimed in claim 1 whereinthe vulcanisation is performed in the presence of at least one compoundselected from the group consisting of mono- or di- carboxylic organicacids having 2 to 20 carbon atoms and polyesters having an acid numberof at least
 20. 5. A process as claimed in claim 4, wherein the amountof the organic acid component is between 0.1 and 5% by weight, based onthe weight of the rubber.
 6. A process as claimed in claim 1, whichcomprises vulcanisation of a rubber composition into which the novolakand the melamine resin have been incorporated at a temperature between80° and 120° C.
 7. A process as claimed in claim 1, which comprisesvulcanisation of a rubber composition containing at least one phenolnovolak selected from the group consisting of phenol-p-tert.-butylphenolnovolaks, phenoloctylphenol novolaks, phenol-nonylphenol novolaks andnovolaks prepared from phenol and cashew nut shell oil.
 8. A process asclaimed in claim 1, which comprises vulcanisation of a compositionwherein the amount of novolak is between 3 to 20% by weight, based onthe weight of the rubber.
 9. An industrial rubber article prepared froma product obtained according to the process claimed in claim
 1. 10. Arubber article as claimed in claim 9 in the form of a tire for motorvehicles.